The present invention relates to a structural metal sheet and a method for forming the same, and more particularly, to a structural metal sheet suitable for use as an outer metal sheet of a monocoque construction (skin-stressed sheet-metal body construction) such as a railway vehicle body having an outer sheet pretensioned in order to increase the buckling strength thereof.
Referring to FIG. 1 showing prior art, a general railway vehicle structure 1 has members which are made thin in order to allow the vehicle to be more lightweight and are arranged to minimize the number of required members. Accordingly, a monocoque construction is adopted, in which the outer metal sheet serves as an important strengthened member. By the way, when a load is placed on the structure 1, a deflection h is produced, and a buckling .delta. arises in the outer metal sheet as shown in FIG. 1.
Therefore, the outer metal sheet is generally pretensioned so as to be prevented from buckling. A conventional pretensioning method is such that the outer metal sheet is subjected to spot-heating-and-quenching after the structure is assembled. Another conventional method is such that the outer metal sheet is pretensioned by pulling the same when it is attached to the frame or expanding the outer metal sheet by heating and securing the same to the frame. However, the spot-heating-and-quenching method is poor in finished appearance and unfavorably increases the number of working steps. On the other hand, the other method inconveniently requires a special-purpose equipment and also increases the number of working steps disadvantagously.